Hello and welcome to JustAnswer!
It depends on how far it is overfilled. If it's overfilled high enough for the geartrain to contact the fluid it will cause the fluid to churn and foam and it will cause slipping. Prolonged driving like that would cause some clutch wear. That being said, the whole Torquflite line of rear wheel drive transmissions like you've got in your truck would take alot more fluid than what the full mark on the dipstick showed before you ran into trouble. It's common to run these about a quart overfull. The extra capacity keeps the fluid a little cooler. This is something done by many trans guys, and something that I picked up from a 30 year Chrysler dealer rear wheel drive trans guy.
Can you explain the whole situation so I can get a better picture of what is going on? The fluid was flushed (hopefully). After that did you have some sort of problem with the trans? Try and get me up to speed on the chain of events and I'll hopefully be able to shed some light on what's going on.
Or are you not having any problems, just thinking that the fluid is overfull and want to make sure nothing is going to be hurt?
I took the truck in for a routine transmission service which included changing the fluid, flushing, and changing the filter and adjusting the bands. I drove the truck, pulling our small (6000 lbs gvw) 5th wheel trailer for approx 1200 miles. I returned to the dealership for a routine engine oil change at which time the tech discovered that the transmission was overfilled and the fluid looked to be burnt. The dealership redid the transmission service with new oil & filter and readjusted the bands and assured me that there was no problem with the transmission.
We then took a trip pulling the holiday trailer and after approx 3000 miles the transmission failed due to a burnt clutch band and debri throughout the transmission.
I think the overfilling episode may have been at least a partial cause of the transmission failure.
It's very hard to say without seeing it in person, or knowing how far it was overfilled. If it had done damage when it was overfilled before it was serviced again, you would have noticed it slipping then. When the fluid gets high enough to contact the geartrain it will foam the fluid, this causes low line pressure and some pretty bad slippage and is what causes clutch damage. You should have definitely felt this going on after leaving from having the first service done. It would also be beneficial to know how far the fluid was overfilled. I would estimate it would take a good 2 inches or more over the full mark before you would get high enough to do damage.
You're welcome, and good luck with your truck!